1. Field of the Invention
The present invention relates to an electromagnetic actuator suitable for use in fuel injection devices for internal combustion engines, and more particularly to an electromagnetic actuator having a position detector for detecting the operating position of a plunger.
2. Description of the Relevant Art
Conventional electromagnetic actuators have a position detector for detecting the operating position of a plunger. The position detector comprises a sensor core mounted on the tip end of the plunger which is axially movable under magnetic forces generated by a drive coil, and a differential transformer comprising primary and secondary coils for detecting the displacement of the sensor core.
Since the position of the plunger is detected by the differential transformer, the differential transformer should not be adversely affected by the magnetic field produced by the drive coil for a higher degree of detecting accuracy. Therefore, the differential transformer and the drive coil must be widely spaced apart, making the position detector large in size.
According to a solution proposed in Japanese Patent Publication No. 1(1989)-25305, the sensor core is made of a nonmagnetic electrically conductive material, and the primary coil of the differential transformer is supplied with a high-frequency current to render the differential transformer less susceptible to the magnetic field, the differential transformer being coaxially disposed in the drive coil.
The proposed position detector is however complex in structure because the differential transformer is housed in the drive coil. Use of the differential transformer to detect the position of the plunger requires that a compensating coil be added to effect detector output adjustments for setting a signal at a reference plunger position to a predetermined value. It is therefore necessary to assemble the compensating coil as well as the exciting primary coil and the detecting secondary coil. Furthermore, an electric circuit associated with the differential transformer is complex.